W.J. McDowell

Sodium and strontium extraction by di(2-ethylhexyl)phosphate: Mechanisms and equilibria☆

Abstract Extraction of sodium and strontium by di(2-ethylhexyl)phosphoric acid (HA or HDEHP) in benzene was examined over the complete loading range from trace concentration to reagent saturation, i.e., formation of the normal salt NaA (NaDEHP) or SrA2 (Sr(DEHP)2). Trace strontium extraction from 0·50 and 4·00 M NaNO3 by mixtures of HA and its sodium salt, NaA, was examined from zero to 100% NaA. Extraction equilibrium curves (organic vs. aqueous concentration), hydrogen ion and reagent-concentration dependences have been examined in all cases, and, for strontium extraction by mixed HA-NaA, sodium-ion dependence was examined. The data indicate that the reaction for sodium extraction by the acid dimer, (HA)2, is Na+ + 2(HA)2·3HA + H+ up to 25% NaA. Additional sodium extraction results in tetramer destruction to yield the salt NaA. The reaction for strontium extraction by (HA)2 is similar: Sr2+ + SrA2·4HA + 2H+. Also similarly, ultimate loading leads to the salt SrA2. Both these salts are polymerized. Trace strontium extraction by mixed HANaA is most readily described by the equation: Sr2+ + (n/y)(aHA·bNaA)y (1/x)(SrA2·(n − 2))(αHA·βNaA))x + iH+ + jNa+, where the extractant is characterized as a mixed, y-fold polymer containing a fraction of HA and b fraction of NaA. The data indicate that strontium extracts as SrA2·4HA at least up to organic-phase compositions of 40% NaA. At higher NaA concentrations the interpretation is less certain but suggests the extraction of strontium into NaA micelles.

Crown ethers as size-selective synergists in solvent extraction systems: A new selectivity parameter☆

Abstract Mixtures of macrocyclic polyethers (crown ethers) and organic-phase-soluble liquid cation exchangers have been found to produce a synergistic effect in the extraction of metal ions. The synergistic effect is size selective; that is, it tends to be greatest for those ions that best fit the crown ether cavity. The mixtures of a liquid cation exchanger and a crown ether also allow metal ion extraction from common mineral-acid anion systems (NO3−, Cl−, SO42−) that would be impossible with the crown ether alone, because of the difficulty of solubilizing those anions in nonpolar solvents. This cooperation makes the use of crown ethers as size-selective coordinators available for process applications. Size selectivity of compounds such as crown ethers may thus become a useful new parameter in designing selective solvent extraction systems.

The sulfate complexes of some trivalent transplutonium actinides and europium

Abstract The sulfate complexes of Eu, Am, Cm, Bk, Cf and Es were investigated by a solvent extraction method using neutral species distribution between a benzene solution of a long-chain amine sulfate and aqueous sulfuric acid-sodium sulfate solutions. Sulfate was the only anion present and the ionic strength varied with the ligand concentration. Activity coefficient effects over the ligand concentration range examined (up to 0·5 M ) were estimated by a Debye-Huckel expression. The use of the Debye-Huckel equation is justified since the γ ± Na 2 SO 4 data can be fitted to 0·5 M and even higher concentrations with the same equation. Evidence for important amounts of the heretofore unreported trisulfate species of these elements was found, but no evidence was found for tetrasulfate species. The equilibrium constants for all of the actinides are close together and show a smooth periodicity as Z varies. The equilibrium constants for europium are close to those for the actinides. Overall formation constants at zero ionic strength for the mono-, di- and trisulfates of each of the elements are given. Percent distribution of each actinide as a function of sulfate ion concentration is shown.

The tetrad effect: The thiocyanate complex stability constants of some trivalent actinides

Abstract A solvent extraction technique, using bis(2-ethylhexyl)phosphoric acid as the extractant, was employed to examine the formation of Am(III), Cm(III), Bk(III), Cf(III), and Es(III) thiocyanate complexes in a NaSCNNaClO 4 medium at ionic strength I = 1·0 M and pH = 2·00. Mathematical analysis of the data led to the conclusion that the mono-, di-, and trithiocyanate species exist in the thiocyanate concentration range studied in this work. Values of the overall stability constants β 1 , β 2 , and β 3 were calculated. As the atomic number of the actinides increased, the β 1 values exhibited a gradual increase in stability, which is consistent with the expected effect on complex stability due to the actinide contraction. The β 2 values decreased rapidly across the series from americium to einsteinium, with a concomitant increase in β 3 . In addition, there was evidence of the tetrad effect in the β 3 values, and this led to the tentative conclusion that the Act(SCN) 3 (where Act = Am, Cm, Bk, Cf, and Es) complexes are of the inner-sphere type. The thermodynamic parameters of the Am(III) thiocyanate complexes were calculated from temperature dependence measurements. The enthalpy and entropy changes were used to aid in the distinction between inner-sphere and outer-sphere complex formation.

Extraction of alkaline earths from sodium nitrate solutions by di(2-ethylhexyl) phosphate in benzene: mechanisms and equilibria☆

Abstract The extraction of the alkaline earths, excluding radium, by di(2-ethylhexyl)phosphate (DEHP) in benzene from 0·50 and 4·0 M sodium nitrate solutions has been examined as a function of pH and DEHP concentration. Curves with a characteristic maximum in the region of pH 5, similar to that already observed for strontium, were found in all cases. Log plots of E M = [ M ] org [ M ] aq . vs 3 pH gave a slope of +2 at low pH indicating exchange of M2+ with 2H+. The relative extractability was Be ⪢ Ca > Mg > Sr > Ba from 4 M NaNO3 and Be ⪢ Ca > Mg ≈ Sr > Ba from 0·5 M NaNO3. Reagent concentration dependences at low pH where DEHP is in the acid dimer form (HDEHP)2 indicate the following number of DEHP monomers associated with each metal atom: From 4·0 M NaNO3: Be, ∼4; Ca, 5–6; Mg, uncertain; Ba, 6; and from 0·5 M NaNO3: Be, uncertain; Ca, 4–5; Mg, 4–6; Ba, 4–6. The lower number always obtained at the lower reagent concentration and the higher number at the higher reagent concentration. The s ame association ratios and also indicated by the organic phase composition NaDEHP Σ DEHP at which maximum extraction occurs.

INTERFACE MECHANISM FOR URANIUM EXTRACTION BY AMINE SULPHATE.

Abstract In the extraction of metal ions by organic solutions of aqueous-insoluble alkyl ammonium salts the mechanism by which the metal species transfers across the aqueous-organic interface cannot be determined by the usual equilibrium studies. In an effort to elucidate this mechanism, interfacial tension of the two-phase system and kinetics of the transfer of35SO4 between organic (di-n-decylamine sulphate in benzene) and aqueous (acid-sodium sulphate) phases were studied. The kinetic experiments were designed so as to offer the possibility of detecting the transfer of anionic uranium species from aqueous to organic phase. Several different aqueous phase situations were examined, covering the range from low sulphate where most of the uranium exists as the uranyl ion to 1·0 M sulphate where most of the uranium exists as the disulphate complex. At the higher aqueous sulphate concentrations (>0·025 M) an increased rate of35S transfer during uranium extraction (at a given organic/aqueous total sulphate ratio) gave strong evidence for an anion exchange mechanism such as Download : Download full-size image (dotted underlines indicating the organic phase). At aqueous sulphate concentration below 0·01 M, the35S transfer rate was decreased during uranium extraction, and the explanation offered for this result suggests transfer by a neutral mechanism such as Download : Download full-size image Thus, it appears that neutral species transfer is possible where aqueous neutral or cationic species predominate and anionic species transfer is possible where aqueous anionic species predominate, with no implication that the two mechanisms are mutally exclusive. These conclusions are supported by calculations of the number of sulphates transferred per uranium based on35S transfer aq → org during uranium transfer aq → org.

Solvent extraction of lithium

Abstract Lithium was quantitatively and selectively extracted from aqueous solutions of alkali metal salts by forming a suitable adduct of a lithium chelate. Thus, the trioctylphosphine oxide (TOPO) adduct of lithium dibenzoylmethane (LiDBM) was readily extracted into dodecane or p-xylene. In systems supported by sodium or ammonium ions, the extracted lithium species had the form, LiDBM·2TOPO. In systems supported by K, Rb or Cs ions, the extracted lithium species was dimeric and had the form, Li2(DBM)2·2HDBM·4TOPO.

Nobelium chemistry: Aqueous complexing with carboxylate ions

Abstract The tendency of divalent nobelium to form complexes with citrate, oxalate, and acetate ions in an aqueous medium of 0·5 M NH4NO3 has been examined by solvent extraction techniques and compared with the complex forming ability of calcium and strontium under the same conditions. In general, for each anion, the complexing tendency of nobelium is between that of calcium and strontium, being more nearly like that of strontium. The data allow the estimation of concentration quotients for the formation of NoHCit and NoOx. The extraction data for nobelium with acetate in the aqueous phase suggest a difference in nobelium chemistry from that of calcium and strontium.

Equilibria in the system: Di(2-ethylhexyl)-phosphoric acid-benzene-water-alkali (hydroxide, nitrate)

Abstract The extraction of the alkali metals from 3M alkali metal nitrate by di(2-ethylhexyl)phosphoric acid (HDEHP or HA) in benzene is examined as a function of HDEHP concentration and of aqueous phase pH. The dependence of extraction on reagent concentration over a wide range of reagent loading ( M A HA ) was interpreted using average degree of aggregation of the 0·1 M reagent in benzene measured in this and earlier work. A single metal organic phase species, MA·3HA, was found to exist up to 25 per cent MA in all cases except Cs (Fr not tested). The cesium system gave evidence of the presence of at least some CsA·4HA or CsA·5HA. At higher fractions of MA, the acid-coordinated species disappears and large aggregates of MA appear. At 100 per cent MA the order of extraction of the alkali metals is: Li > Na > K > Rb > Cs. However, the extractabilities of K, Rb, and Cs are very close together and Cs becomes more extractable than K and Rb at low fractions of MA.

Sodium and strontium extraction from sodium nitrate solutions by n-octane solutions of a branched aliphatic monocarboxylic acid: Mechanisms and equilibria

Abstract Sodium and strontium are extracted from 5·0 M NaNO3 solutions by n-octane solutions of a highly branched aliphatic carboxylic acid (RR′R″CCOOH), a fraction distilled from a developmental mixture “Versatic Acid 1519”. From the dependence of the extraction on pH and extractant concentration, from extractant and extract aggregation in the organic phase and from i.r. absorption studies of the complexes present in the extract, the most probable extraction reactions are Na + +1·5( H A) 2 ⇌ Na A·2 H A+ H + Na + +2( H A) 2 ⇌ Na A·3 H A+ H + Sr 2+ +2( H A) 2 ⇌ Sr 2 ·2 H A+ 2H + Sr 2+ +2·5( H A) 2 ⇌ Sr A 2 ·3 H A+ 2H + The carboxylic acid extraction of sodium and strontium from sodium-salted systems is, in several ways, similar to di(2-ethylhexyl)phosphoric acid (HDEHP) extraction of these ions. Similar salt-plus-coordinated-acid complexes are formed, thus resulting in the curves of strontium extraction coefficient vs. pH having the same general shapes and well defined maxima. The maximum strontium extraction coefficients and separation factors from sodium are almost 100 times greater with this carboxylic acid than with HDEHP. Maximum extraction with the carboxylic acid occurs, however, at pH 8·5–9·5 while with HDEHP it occurs in the pH range 5–6. Thus, the advantage in extraction coefficient may be limited to the more basic ions.